refactor(kpi): K4/K10/K12 review fixups — test data-race + faulted-tick liveness, dead-branch/unused removal, NaN-guard assertions, value clamp + doc

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/KpiTrendChart.razor.cs

@@ -82,9 +82,6 @@ public partial class KpiTrendChart
     /// <summary>Exactly one sample — show the single-sample note, not a polyline.</summary>
     private bool HasSingleSample => IsAvailable && Series.Count == 1;
 
-    /// <summary>Available + ≥1 point but not a full chart, or unavailable / empty.</summary>
-    private bool ShowPlaceholder => !IsAvailable || Series.Count == 0;
-
     /// <summary>
     /// Stable Playwright hook: <c>kpi-trend-&lt;slug&gt;</c> where the slug is the
     /// title lowercased with each run of non-alphanumerics collapsed to a single
@@ -175,13 +172,17 @@ public partial class KpiTrendChart
             var p = Series[i];
 
             // X — by time fraction, or even index spacing when all timestamps equal.
+            // n >= 2 is guaranteed by the HasChart precondition (Series.Count >= 2),
+            // so the (n - 1) divisor is always ≥ 1 and the n == 1 arm is unreachable.
             double xFrac = timeSpanTicks > 0
                 ? (p.BucketStartUtc.Ticks - firstTicks) / timeSpanTicks
-                : (n == 1 ? 0.0 : (double)i / (n - 1));
+                : (double)i / (n - 1);
             var x = PadX + (xFrac * plotW);
 
             // Y — baseline at 0, top at max. Flat at baseline when max == 0.
-            double yFrac = max > 0 ? p.Value / max : 0.0;
+            // Clamp to non-negative so a stale negative Value cannot push a point
+            // above the baseline or outside the viewBox.
+            double yFrac = max > 0 ? Math.Max(0, p.Value) / max : 0.0;
             var y = baselineY - (yFrac * plotH);
 
             if (i > 0)